Methods for improving the properties of refractory metals



United States ate'nt METHODS FOR Ill/IPROVING THE PROPERTIES OFREFRACTORY METALS Maurice R. Commanday, Los Angeles, and James-R.

Darnell, Reseda, Calif., assignors, by mesne assignments, to ChromizingCorporation, a corporation of Delaware No Drawing. Filed July 10, 1959,Ser. No. 826,112

8 Claims. (Cl. 148-131) This invention has to do generally with animproved process for improving the properties of metals by their surfacetreatment, and relates particularly to the conditioning of therefractory metals tungsten and molybdenum as and for the purposes hereinexplained.

The refractory metals molybdenum and tungsten are subjecttocontamination when heated in air or other non-inert gases to thetemperatures required to relieve stresses induced by mechanical working(in the 1700 to 2000 F. range). It is generally believed that thiscontamination greatly reduces the important property of ductility attemperatures around the normal ambient range. In addition, theserefractory metals are notoriously reactive with air at annealingtemperatures, to the extent that some means must be taken to preventcontact with air or other oxidizing gases; otherwise completedestruction of the material results. Presently, and heretofore, stressrelief annealing of these materials is understaken in furnaces in whichhigh purity hydrogen or argon atmospheres are maintained; Theseatmospheres areeither reducing or neutral to a considerable degree andare capable of preventing oxidation and, in the instance of hydrogen asapplied to annealing of molybdenum,'surface oxides are reduced at thistemperature. p

The construction, maintenance and operationof'large furnaces employingthese special atmospheres is both expensive and, in the case ofhydrogen, hazardous. One object of this invention is to eliminate theneed for complex, expensive equipment, and is a further object of thisinvention to eliminate the hazards associated with the use of hydrogenatmosphere furnaces.

It appears that room temperature ductility of many metals is directlyrelated to freedom from contamination, and that the impuritiesresponsible for-thisapparent lack of ductility may exist in the form ofinterstitial compounds such as oxides, nitrides, etc. 'I nterstitialcontamination can be greatly alleviated through the use of specialannealing atmospheresthat employ mixtures of hydrogen with halidegases,but the employment 'of'these highly specialized annealing atmospheresis' expensive. and cumbersome. In addition, since all'halides" arehighly toxic, obvious hazards are involved.

A further purpose of this invention is to provide halide gas annealingatmospheres without the use of expensive equipment and under conditionsassuring complete safety to working personnel and equipment.

As illustrative, for purposes of presenting the invention in itsapplication to the refractory metals molybdenumand 1 tungsten, we mayrefer to a material such as .5 titaniumbalance molybdenum alloy, arefractory metal in current popular use, the interstitial contaminationin which can be measured in terms of its ductile-to-brittle bendtransition temperature. If one attempts to bend a given piece of thismaterial at a temperature below the transition temperature for thatparticular piece, it will break with little or no plastic deformation asthough it were glass or ceramic. If the same attempt to bend is made ata temperature above the transition temperature for the piece ice 2 inquestion it will bend and exhibit reasonable plastic deformation. Thetemperature at which this change takes place (i.e., fromductile-to-brittle bend characteristics) is extremely well definedwithin a matter of four or five degrees Fahrenheit. There is no gradualchange, but rather a very abrupt one.

of tungsten This powdered mixture is-in the 'nature'of u a pack whichmay contain an additional powdered'co'm t y Studies have been made ofbend transition temperatures for various lots of .5% titanium-balancemolybdenum alloy from various suppliers, and values have beenestablished ranging from minus 30 F. to plus 200 F. In addition,considerable variation in transition temperature has been observed onpieces cut from the same sheet. This can be understood in terms of avariance in the distribution of interstitial contamination over thesurface of a given piece of material. This is a particularly awkwardcondition from the point of view of fabrication techniques such asbending, forming, shearing, etc., since the variation in transitiontemperature occurs Within the normal room temperature range. For thisreason, shop personnel may find that they may safely shear and bend apiece of .5% titanium-balance molybdenum alloy at room temperature, orthe specimen may break, depend-- ing upon its specific transitiontemperature (i.e. purity). This has led to the adoption of the practiceof hot'bending and hot shearing, which is expensive and awkward, and, infact, may add to the contamination. Moreover, many refractory metalitems of manufacture contemplated would be far more valuable if theycould be considered ductile over a broader range of temperatures. Suchitems may be subject to temperatures below freezing as well as in therange of 3000 F. and higher. Obviously, any accidental or service impactloads imposed on a refractory metal part at temperatures below itstransition temperature would be likely to cause complete failure.

it has been shown that the transition temperature of .5%titanium-balance molybdenum alloy sheet can be signficantly reduced bythe order of 20 degrees Fahrenheit by removal of the outer .005" ofmaterial by grinding. It is generally accepted that the 'portion ofmaterial most clearly responsible for loss of ductility is the outerlayer, which is evidently more heavily contaminated, It

is a current practice, therefore, to surface grind material prior toworking. This operation is both costly in the grinding as well asin theloss of expensive material; frequently of the original supplied weightof the'mate:

rial. 1

The present invention aims to so purify the outer layers of material asto eliminate the necessity of surface grind It appears possible that theapplication of, this.

these metals-alloyed with smaller percentages of other metals such astitanium,-nickel, chromium and others depending upon particularproperties desired in the'alloy'; i

As' generally contemplated, the present process involves heatingthem'etal (molybdenum, tungsten or their alloys) in contact with apowdered mixture of an ammonium halide salt, and a powdered quantity ofthe same refractory metal, i.e. powdered molybdenum where the metal 1 ismolybdenum or an alloy thereof, or powdered tungsten where the metalbeing treated is tungsten or an alloy ponent serving as an inert soliddiluent. I

The only equipment required for stress relief'anneal ing of therefractory metal consists of an appropriate furnace, such as a simplegas-fired, open-fire or semimuflle type furnace, and a treatment retort,chamber or box of the kind well known in the art of diffusing metalssuch as chromium, onto the surfaces of metals being treated. See e.g.Samuels Patent No. 2,536,774. The metal to be treated is packed in theretort box in surface contact with a finely powdered compound comprisingapproximately 50% to 95% by weight of an inert diluent such as clay,pumice, talc, silica or preferably tabular alumina (Alcoa T-61) 60 meshand finer; to 50% molybdenum or tungsten (as the case may be) powder 60mesh and finer, and a small quantity, .05 to .5 (more may be used but tono advantage) of ammonium halide salt such as ammonium bifluoride,ammonium bromide, ammonium iodide or ammonium chloride. This powderedpack material is compacted about the metal being treated so as toexclude as much air as possible. The treatment retort may be of theknown box type having a top sealed to the body of the box by means of afusible seal such as powdered glass, which melts during the heatingperiod to allow gases to bleed out of the box, and which freezes uponremoval from the furnace so as to prevent the ingress of air uponcooling. The loaded and sealed retort is introduced into a furnaceheated to a temperature that may be in the range of about 1700 F. to2200 F., or higher (typically about 1850 F.), and held there for aperiod of time ranging from four to twenty-four hours. During theinitial period of heating, the ammonium halide salt decomposes to formcracked ammonia and halogen gas. The halide gas reacts preferentiallywith the powdered molybdenum or tungsten, since the powder affords agreater surface area than the metal being treated, so that a mixture ofmolybdenum or tungsten halide and cracked ammonia pervades the treatmentzone.

The exact nature of the ensuing chemical reactions are not fullyunderstood. However, we believe that the cracked ammonia acting inconjunction with the molybdenum or tungsten halide reduces the oxidesand forms gaseous compounds with the unwanted interstitial contaminantsat or near the surface, leaving a purified surface. This purifiedsurface then is believed to set up an unstable zone into whichsubsurface interstitial contami' nants diffuse to be reacted and removedby the aforementioned gaseous mixture. The mechanism may be analogous tothe reaction occurring in decarburization of carbon steel in which,through a surface chemical reaction, the solid compounds are caused tobe removed from zones near the surface. 1

The following examples are illustrative of the invention:

Five specimens of cold worked .5 titanium-balance molybdenum sheet metalapproximately 0.060" thick were treated for 8 hours at 1850 F. incontact with packs composed as follows:

Specimen #1:

No treatment (pack) Specimen #2:

Pack mixture 0% molybdenum approximately 100% tabular alumina, plus 3%ammonium bifluoride. Specimen #3:

Pack mixture 1% molybdenum, approximately 99% tabular alumina, plus .3%ammonium bifluoride 6 4 Specimen #4:

Pack mixture 3% molybdenum, approxnnately 97% tabular alumina, plus .3%ammonium bifiuoride. Specimen #5:

Pack mixture 5% molybdenum, approximately tabular alumina, plus 3%ammonium bifiuoride.

After treatment, the pack-treated specimens were removed from the retortin which they had been allowed to cool in still air, and visuallyexamined. Specimen #2 was observed to be duller than the remainingtreated specimens which were all bright in color. Subsequently, thespecimens were sectioned transversely and the following Knoopmicro-hardness determinations were made;

Nsuo Specimen No. 0.003" 0.000"

" Distance within :1 treated surface as measured on a cut for.

Softness of the metal is generally a function of its purity, i.e.freedom from interstitial contamination. From these data, it is observedthat increasing softness follows increases in the molybdenum content ofthe pack.

We claim:

1. The method for surface decontamination and improvement of theproperties of a refractory metal of the group consisting of molybdenumand tungsten and alloys of molybdenum and tungsten wherein the alloysconsist essentially of molybdenum or tungsten, that includes heating themetal within a closed zone in contact with a powdered uniform mixturecomprising about 50 to 95 percent (by weight) inert diluent, about 5 to50 percent of the same refractory metal in powder form, and about 0.05to 0.5% ammonium halide, for a period of about 4 to 24 hours at atemperature sufiicient to dissociate the halide and combine the releasedhalogen with the powdered refractory metal.

2. The method according to claim 1, in which the diluent is tabularalumina.

3. The method according to claim 1, in which the halide is ammoniumbifluoride.

4. The method according to claim 1, in which said temperature is betweenabout 1700 F. to 2200 F.

5. The method according to claim 1, in which the refractory metal isessentially molybdenum.

6. The method according to claim 1, in which the refractory metal isessentially molybdenum and the halide is ammonium bifluoride. 7. Themethod according to claim 1, in which the said same metal is essentiallymolybdenum, the. halide is ammonium bifluoride and said temperature isbetween about 1700 F. and 2200 F.

8. The method according to claim 1, in which the refractory metal isessentially tungsten.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD FOR SURFACE DECONTAMINATION AND IMPROVEMENT OF THEPROPERTIES OF A REFRACTORY METAL OF THE GROUP CONSISTING OF MOLYBEDENUMAND TUNGSTEN AND ALLOYS OF MOLYBDENUM AND TUNGSTEN WHEREIN THE ALLOYSCONSIST ESSENTIALLY OF MOLYBDENUM OF TUNGSTEN, THAT INCLUDES HEATING THEMETAL WITHIN A CLOSED ZONE IN CONTACT WITH A POWDERED UNIFORM MIXTURECOMPRISING ABOUT 50 TO 95 PERCENT (BY WEIGHT) INERT DILUENT, ABOUT 5 TO50 PERCENT OF THE SAME REFRACTORY METAL IN POWDER FORM, AND ABOUT 0.05TO 0.5% AMMONIUM HALIDE, FOR A PERIOD OF ABOUT 4 TO 24 HOURS AT ATEMPERATURE SUFFICIENT TO DISSOCIATE THE HALIDE AND COMBINE THE RELEASEDHALOGEN WITH THE POWDERED REFRACTORY METAL.